1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, and specifically relates to an image forming apparatus including a developing device with a two-component development system,
2. Description of Related Art
In general, an image forming apparatus using electrophotographic process technology (such as a printer, a copy machine, a fax machine) is configured to irradiate (expose) a charged photoconductor with (to) laser light based on image data to form an electrostatic latent image on a surface of the photoconductor. The electrostatic latent image is then visualized by supplying toner to the photoconductor (image bearing member) with the electrostatic latent image formed thereon, whereby a toner image is formed. The toner image is directly or indirectly transferred to a sheet, followed by heating and pressurization for fixing. Consequently, an image is formed on the sheet.
Development systems for forming a toner image on a photoconductor include a one-component development system using only toner as a main component of a developer and a two-component development system using toner and carrier as main components of a developer. In the two-component development system, toner and carrier are mixed and stirred to triboelectrically charge the toner. In order to stably charge the toner, it is ideal that there is no change in surfaces of particles of the carrier.
In a developing device with the two-component development system, toner is consumed in a developing process while carrier is not consumed but remains in the developing device. Thus, mechanical stress and thermal stress due to contact with the toner are accumulated on the carrier, and the carrier particles surfaces are contaminated by adhesion of toner. Temporal degradation of carrier reduces the amount of charge on the toner, resulting in image quality deterioration such as fogging.
To avoid the foregoing problem, a degraded developer in a developing device is periodically replaced. Furthermore, since toner and carrier in a developing device are different from each other in degradation rate, developing devices configured so as to separately supply toner and carrier have been proposed (see, for example, Japanese Patent Application Laid-Open No. 2005-250347 (PTL 1)).
More specifically, PTL 1 discloses a developing device including a carrier supply section that includes a plurality of resupply rollers each including measuring recess portions formed at a peripheral surface thereof, in which carrier put in the measuring recess portions is supplied to respective developing sections by the resupply rollers being rotated. In other words, in the developing device disclosed in PTL 1, carrier is distributed and supplied to a plurality of developing sections by a plurality of resupply rollers.
An image forming apparatus may suffer adherence and/or deposition of carrier to/on members due to changes in chargeability and flow ability of the carrier by the environment (in particular, humidity) inside the apparatus. For example, in the developing device described in PTL 1, carrier may adhere to the measuring recess portions of each resupply roller, which serves as a carrier distributing section, and/or junction sections to join a toner resupply channel positioned immediately below the respective resupply rollers.
However, the developing device in PTL 1 includes no means for preventing adherence of carrier, and thus, cannot prevent temporal adherence and deposition of carrier, resulting in failure of stably resupply of a fixed amount of carrier to each developing section.
As described above, if a fixed amount of carrier cannot be supplied to respective developing sections with good accuracy, the balance between the toner amount and the carrier amount in the developing sections may be lost and the toner may unevenly be charged, which may result in deterioration in image quality.